It is known that in this type of resonator the operating frequency depends directly on the thickness of the active crystal (U.S. Pat. No. 3,694,677). To achieve frequencies as high as those mentioned above, the thickness of the resonant element has to be extremely small (of the order of a few microns up to 100 microns), such that mechanical fabrication methods are wholly unsuitable.
Furthermore, handling of the elements is a very delicate operation and during use shocks can be fatal and cause the element to break.
This is why it is proposed in the abovementioned U.S. patent on the one hand to provide the resonator element with a reinforcing surround, which is considerably thicker than the resonator itself, and on the other hand preferably only to use non-mechanical fabrication methods such as chemical or ion etching at least in the last stage of the process leading to the desired thickness of the resonant element. Although these precautions can resolve to a certain extent the mechanical problems mentioned above, there nevertheless remain problems residing in the coupling which is between the resonator element and the reinforcing surround and which is too large. This leads to parasitic resonances and generally to a mediocre resonator characteric.
U.S. Pat. No. 4,135,108 describes a quartz resonator of the typein which the quartz is mounted as a dielectric between the electrodes of a capacitor, these electrodes being deposited on plates which hold the resonator in place. To this end, the resonator may comprise a peripheral ring wedged between the plates and separated from the resonator element proper by thinned-down portions which may take the form of simple connecting bridges extending radially between the peripheral ring and the resonator element.
The ring, the thinned-down bridges and the resonator element are of one piece and the shape of this whole unit is obtained by techniqes such as abrasion, chemical attack or the like. In certain cases this unit may be of uniform thickness throughout, with the exception of the connecting bridges, the main surfaces being parallel. Adjustment of the frequency of the resonant element may be obtained by the removal of material, but such a precise machining operation then involves all the main suface or surfaces of the unit comprising the ring, the bridges and the resonant central portion, in such a way that it entails a general mechanical weakening of this unit, including the outer ring. This latter can thus no longer act as a mechanical reinforcement and serves ultimately only as a fixing member. Although this arrangement leads to better decoupling with respect to the support, it is still fragile and withstands handling with difficulty. Furthermore, if the ring is relatively thin the reduction in the thickness of the central portion cannot be as great as could be desired, taking into account the high resonance frequencies to be obtained.